Since a pacemaker was first introduced by Furman and Rovinson in 1958, the pacemaker has been used as an important device for treating patients with bradyarrhythmia. Recently, a pacemaker is usually used in treatments for arrhythmia such as complete atrioventricular block, high degree atrioventricular block, and sinus node dysfunction accompanied by symptoms. A treatment using a pacemaker is a method that artificially provides an electrical stimulus when an electrical stimulus is not normally transmitted to a heart.
FIG. 1 is a view of a conduction system of a heart, in which (A) shows a flow in a conduction system, (B) shows a waveform in an electrocardiogram, and (C) illustrates the relationship between a conduction process and a waveform.
Referring to FIG. 1, according to the conduction of the heart, an electrical stimulus is transmitted to the overall ventricles through a conduction pathway after passing through a sinoatrial (SA) node, an atrioventricular (AV) node in atriums and then passing through the bundle of His and a bundle branch in ventricles. Transmission of stimulus in a ventricle is made by a His-Purkinje system.
In an electrocardiogram, a QRS-complex is generated by a depolarization process of ventricular muscles, the first downward wave following a P-wave is called a Q-wave, the first upward wave is called an R-wave, and the downward wave following the R-wave is called an S-wave. The width of the QRS means the time taken for electricity to be conducted throughout the entire ventricles. The width of the QRS is within about 0.12 seconds (around about 90 ms) in a normal state, but when it is 0.12 seconds or more, it indicates the presence of an interventricular conduction defect including blocks.
A pacemaker is composed of a generator and a lead. The generator supplies power and includes a computer, so it supplies power, if necessary, or suspends the power by checking the state of electricity flowing into a heart through the lead. The lead transmits electricity from the generator to the heart.
FIG. 2 shows the treatment performed at present by a pacemaker.
According to a common treatment that is performed by a pacemaker at present, the tip of the lead of a pacemaker is inserted and fixed in the apex of the right ventricle (RV apex) of ventricles and then electrical stimulus is provided. This is called right ventricular apical pacing (RVAP).
As for RVAP, the electrical stimulus at the RV apex is not transmitted through the conduction system that is a special tissue structure for quickly transmitting electrical stimulus in a ventricle, but transmitted through cariomyocytes of the ventricle that relatively slowly transmit electrical stimulus, so it takes long time for the electrical stimulus to spread through the entire ventricle. It means that the QRS width increases in an electrocardiogram and is called “Wide QRS”. That is, the QRS passing through the RVAP is about 160 ms, which is considerably delayed as compared with 90 ms for a normal case.
The Wide QRS causes non-uniformity of motion of ventricles, that is, ventricular desynchronization, so it causes a side effect of loss of the ventricular function. Many studies for obtaining narrower QRS by giving electrical stimulus to portions close to a conduction system at an interventricular septum have been conducted to overcome the side effect.
Representatively, a method of positioning the tip of a pacemaker lead at a right ventricular basal septum and applying electrical stimulus around the conduction system has been attempted. This is called right ventricular septal pacing (RVSP). The RVSP is most usually used at the interventricular septum of a right ventricular outflow tract (RVOT).
The RVSP theoretically compensates for the defects of the RVAP, but in the actual operation it is difficult to accurately position the lead of a pacemaker at the interventricular septum around the RVOT and the lead may be separated or moved, so the operation itself is difficult and accordingly it is not generally used. The RVSP has another characteristic that positions the lead tip at an interventricular septum, but stimulates not the inside, but the outer side of the interventricular septum, and it is known that the RVSP is less effective than the method of stimulating the endocardium or the center of an interventricular septum.
Another method of obtaining a narrower QRS is applied to a case when a patient with heart failure accompanied by ventricular insufficiency has a wide QRS in an electrocardiogram. This method uses two leads, and positions a lead at an RV apex and applies electrical stimulus and positions the other lead at a left lateral vein and applies electrical stimulus to a side of the left ventricle. That is, it is a treatment for securing a narrower QRS by simultaneously applying electrical stimulus to the RV apex and the side of the left ventricle. It is called “Cardiac Resynchronization Therapy (CRT)”.
The CRT is known as a very effective and remarkable treatment when a patient with heart failure has LBBB (left bundle branch block), etc. However, the CRT has a defect that it has to use two leads for stimulating ventricles in order to obtain a narrower QRS.
According to the treatments used up to now, if it possible to directly apply electrical stimulus to the interventricular septum where the conduction system of ventricles are positioned when applying electrical stimulus to the ventricles, it is possible to obtain a narrower QRS and the transmission direction of the electrical stimulus can apply biological electrical stimulus, so it is possible to overcome the problems with the RVAP and expect good effects from some patients who need CRT.
According to various recent studies on animals, it has been reported that it is possible to further compensate for the defects of the RVAP by directly applying electrical stimulus to the interventricular septum and it is also possible to give help for ventricular insufficiency that needs the CRT. Intraseptal pacing that can apply direct electrical stimulus to an interventricular septum has been attempted, and methods by forcibly positioning the lead of a pacemaker into the interventricular septum directly through the left ventricle from the right ventricle have been disclosed in US2010/0298841 and US 2013/0231728. These methods have high invasion depth that causes an artificial loss of interventricular septum between the left and right ventricles, have a high possibility of tearing surrounding tissues during the operation, and have a high possibility of causing an embolism due to air or blood clots. Further, the methods have many dangers and limits, for example, it can locally approach the middle portion or the apex of ventricles rather than the base which is preferable.
Accordingly, it is required to study a method that can more safely and simply obtain a narrower QRS by directly applying electrical stimulus to the conduction system of a ventricle.